More powder,same velocity

[Downwindtracker2]

I have been doing load development for a couple odd calibers,264WM and 338.06.The rifles are rebarreled FNs ,one with a Douglas and the other a Shilen,both long throated,longer than then the 30.06 length boxs anyway.Their isn't a lot of data for them,so I have been shooting the groups over a Chrony.One time with each rifle,increasing the charge by a grain,<2%,The next loading shot the same velocity.What's going on here?

[PJgunner]

Originally posted by Downwindtracker2
I have been doing load development for a couple odd calibers,264WM and 338.06.The rifles are rebarreled FNs ,one with a Douglas and the other a Shilen,both long throated,longer than then the 30.06 length boxs anyway.Their isn't a lot of data for them,so I have been shooting the groups over a Chrony.One time with each rifle,increasing the charge by a grain,<2%,The next loading shot the same velocity.What's going on here?

Methinks you have passed the max load for your rifle (s).
Let me try to explain. Every powder has a pressure range that within that range it is efficient. If you go below that range, velocity may become erratic, or in the case of the slow burners, you can get a pressure excursion phenomenon, and that's bad as the gun can blow up. However, going the other way, once you have reached the topmost portion of that efficient pressure range, strange thing happen. velocity can either take a bigger increase, stay the same (as in your case) or even be less that the previous load.

Let's say that your cartridge has a max load of 60.0 gr. of powder X. As you approach that top load, velocity stops increasing in a linear mode at say 58.5 gr. That is most likely the max load for that particular rifle. Assuming a 50 FPS increase for each grain increas until you get to 58.5 gr. and all of a sudden, velocity increases only by 20 FPS, or increases, there's no set rule, then that 58.5 gr. load is the absolute max and realy should be decreased by at least one to one and a half grains.

The fact that there was no increase in velocity convinces me of what I've just posted. Be careful.
Paul B.

[BILLY D.]

DW2

PJ IS CORRECT. IN BALLISTIC TERMS THIS IS CALLED THE BREAKOVER POINT. HIS TREATISE IS CORRECT.

["yote"]

As a former Jet engine mech, the term that we used was;
You have reached the "Saturation Point". Pour all the fuel that
you want into it . It's not going to go any faster.

[Jack]

Downwind, I agree with the others.
I've seen a ML do this, too, to the extent that adding more powder made the velocity decrease. What was going on was that the surplus powder wasn't igniting, so it was acting as just more stuff the burning powder had to push out the barrel.
With smokeless,at the much higher pressure, all the powder'll burn, which may not be so good

[Dutchboy]

Right-o, the extra powder is not turned into gas. You might find, however, that by going to a more vigorous primer (Fed 215 or WLRM), or a mild to heavy crimp, you CAN get that extra powder to burn.

Both have the effect of changing the burn rate of the powder.

As always, when changing components or procedures, work back up. FWIW, Dutch.

[Rocky Raab]

Let me emphasize that last sentence of Dutchboy's...

If you change anything, back off a generous amount before you shoot again. That's TRIPLY important with a load that may already be past a safe maximum, as yours might be.

With smokeless powder, though, it's not a matter of the extra powder not burning that causes a loss of velocity.

What happens is that the pressure rises SO high that the bullet obturates more and more with each charge increase. As it fits tighter and tighter in the bore, the force it takes to move it goes up almost exponentially. So the velocity drops off.

But the pressure SOARS!

Using a chronograph is a safety measure when working up charges. The moment you see a dropoff or even a smaller increase in velocity, you are either at or past maximum for that gun and load combo.

[MarkL]

Rocky's explanation is the first one I've heard that makes any sense regarding this phenomenon.

I can't argue with the observed behavior, but I've never bought into the theory that the powder magically knows what the maximum rated pressure is for a given caliber and starts behaving non-linearly at that pressure. The same powder might be used in various cartridges with max pressures ranging from 50K to 65K.

What Rock wrote makes a lot more sense. However, it seems to me that if excessive obturation is causing this behavior, pressure must be very high indeed, and/or the bore must be rough. The pressure at which this occurs need not have anything to do with the max rated pressure for the rifle/cartridge, and is probably above the max rated pressure for ALL cartridges.

Also, it seems a lot would depend on how hard the bullet is. Maybe you would not see this phenomenon at all with a solid copper bullet. Not that it can't obturate, but you would be seeing unmistakable pressure signs and/or velocities long before you reached the necessary pressure. Maybe.

[denton]

I think that the opinions posted are quite correct, but there is one other possibility that needs to be considered.

If you take run-of-the mill brass, fill it precisely with identical amounts of powder, and use unselected bullets, you will still get fairly significant random variation in muzzle speed. The typical standard deviation is perhaps 30 fps, which is about 1%, which means that 95% of all shots will be within 60 fps, which is about 2%.

If your powder is going up in 2% steps, your muzzle speed is only going up in about 1% steps, and that is well within the normal, random error of the process. A single cartridge in such a series, that fails to show an increase in speed, means next to nothing.

If the pattern continues to show itself, with several shots, then, indeed, you're at saturation, and more fuel won't make the bullet go faster. This can also be detected by the presence of large wood splinters in your left forearm.

["yote"]

Downwind, I don't know what loads you are using, but I have both a .264 WIN and a 338-06. In the 264, 71grs of H4831 with a 95g HNDY VMAX gives me a solid 3650 fps and 3700 on a warm
day. The 125 NOS PART with 65grs of H4831 gets me 3375fps.
And the 140g NOS PART loves 60.5grs of H4831 at a solid 3000fps. H4831Is the powder in the .264 WIN.
As for the 338-06,H 414 is the final word as powder of choice for all bullet weights.
IMHO.

[earschplitinloudenboomer]

OOOOOOOOOOORRRRRRRRR...you've changed your PRESSURE CURVE, by increasing your powder charge just enough to overcome the inertia of moving the bullet just enough to make room for the burning gases to expand more quickley...UNTIL the bullet takes the rifling, then slowing down, allowing the pressure to build once again. Chamber pressure effects velocity, but not necessarily proportionately. If you don't feel that your load should be maxed out, if none of the signs of excessive pressure are present...sticky bolt, bulged brass, enlarged primer pocket, cratered primer etc...then it may not be maximum. I would still research for loading data, and back off on the charge until the cause of this deviation is determined.

["yote"]

Something that has not been mentioned; How long is the barrel
on your 264 WIN ? The 264 MUST have a 26" tube. Anything less
will only result in 270 WIN velocities. You can shove all the powder
you want down the 24" tube of a 264 and get nothing but a 270
WIN. Give it 2 more inches and you have a whole new ballgame!

[Dutchboy]

Rocky, that's a new one for me. Do you have any references on that theory you could direct me to? TIA, Dutch.

[Rocky Raab]

Let me look, DB. I'm positive that it will be included in Ken Howell's new book, if it ever sees the light of day (he's very ill and he self-publishes his books...).

He and I have discussed this phenomenon at length, and we agree on it. The basics are well known.

ALL bullets obturate on firing. The pressure applied to the base of the bullet has to act against the bullet's inertia, plus the retarding effect of engaging the rifling. The result is a shortening of the bullet, accompanied by the resultant "fattening" effect.

This is normally a good thing. That's what seals the bore and keeps the powder gases behind the bullet.

The harder the bullet is pushed (i.e., the higher the initial pressure spike) the more obturation that occurs. In rifles with long (or worn) throats and rough or eroded rifling, the problem is worse.

So, with extreme pressure, the bullet can obturate several thousandths larger than bore diameter in the time it moves from the case neck until it hits the rifling. It then has to be swaged back down by the rifling. The energy that is required to do this can be extreme. Coupled with the drastically increased friction, and the net effect can be BOTH reduced velocity and much higher pressures.

[Dutchboy]

OK, now I'm with you. I was thinking you were talking about increasing the bullet diameter as it was going down the bore. That was a new concept.

Pressure spikes like you describe are very well documented and reasonably well understood.

What they are not, however, is consistent. Sometimes you get a spike, sometimes you don't. As has been previously discussed here, that's why the 243 Win and 7mm Mag have been downloaded lately: unusually large pressure swings.

In the case described here, if I am interpreting the post correctly, there is a consistent plateau in velocities. This has also been documented repeatedly, but, in my opinion, it cannot be explained by varying obturation pressures. If pressure spikes were the explanation, it would be more random. Consistent plateauing of velocity is, again in my opinion, consistent with incomplete ignition.

Pushing beyond the plateau is rather risky: if you create a situation where all that powder does ignite early in the ignition sequence, it is equivalent to adding several grains to your charge all at once.

Fortunately, regardless of the cause (inconstent obturation or incomplete combustion), a more vigorous primer and/or a crimp will both lead to the same result: more consistency. JMO, Dutch.